In the pursuit of increasingly efficient and/or new chemical transformations, homogeneous transition metal catalysts are proving to be invaluable components of the synthetic chemist’s toolbox. Notwithstanding the many important contributions made to the area of synthetic chemistry utilizing other transition metal catalysts, palladium-catalyzed cross-coupling techniques have been demonstrated to allow for a plethora of otherwise very difficult or even impossible bond forming reactions to be realized. In this context, appropriately designed ancillary ligands, which upon binding to a metal center can influence metal-centred reactivity, have played an essential role in the advancement of palladium-catalyzed cross-coupling reactions. This thesis describes a multi-faceted approach to the identification of effective ligands for the palladium-catalyzed construction of (sp2)carbon-nitrogen and -oxygen bonds. A new series of P,O-DalPhos ligands were developed and applied in the synthesis of of N-substituted indoles via tandem palladium-catalyzed cross-coupling/cyclizations of ortho-alkynylhalo(hetero)arenes with primary amines. Notably, one P,O-DalPhos variant, OTips-DalPhos, was demonstrated to offer the broadest known substrate scope in this important class of transformations, affording a variety of structurally diverse indoles and related heterocyclic derivatives in high yields. Also described herein is the identification of the previously reported ligand BippyPhos as an extremely robust and versatile ligand in both palladium-catalyzed carbon-nitrogen and -oxygen cross-coupling applications. Indeed, the use of a Pd/BippyPhos catalyst enabled the cross-coupling of a range of (hetero)aryl (pseudo)halides with primary and secondary amines, NH heterocycles, amides, ammonia and hydrazine, with representative examples being accommodated in air. The unprecedented scope of the Pd/BippyPhos catalyst in carbon-nitrogen cross-coupling allowed for the development of two novel one-pot, two-step syntheses of N¬-aryl heterocycles from ammonia, ortho-alkynylhalo(hetero)arenes and (hetero)aryl halides through tandem N-arylation/hydroamination reactions. A marked selectivity profile was also observed for the Pd/BippyPhos catalyst and successfully exploited in the chemoselective monoarylation of substrates featuring two distinct and potentially reactive NH-containing moieties. Finally, Pd/BippyPhos mixtures served as robust and efficient catalysts for the hydroxylation of a range of (hetero)aryl halides and ortho-alkynyl(halo)heteroarenes to form phenols and phenol-derived heterocycles.

Cyclometalated phosphine-based PNP and PCP ‘pincer’ complexes of the platinum group metals have been the subject of intense research in recent years, owing to the remarkable stoichiometric and catalytic reactivity exhibited by such complexes. With the goal of discovering new metal-mediated reactivity patterns and extending the versatility of metal pincer chemistry, significant effort has been devoted to the synthesis of structurally and/or electronically related systems where strategic alterations have been introduced to the pincer ligand architecture, including variation of the central and peripheral donor fragments, as well as the ancillary ligand backbone. In this context, the synthesis and study of Ru and Pd complexes supported by pincer-like tridentate ancillary ligands that feature a central anionic phosphorus ([NPN]) or silicon ([PSiP]) donor in the pincer ligand backbone are described herein. The decreased propensity for forming ?-bonds to P was anticipated to lead to a higher degree of electronic unsaturation in complexes supported by tridentate phosphido ligation relative to structurally related metal amido (M-NR2) species. In the case of [PSiP] ligation, the reduced electronegativity of Si relative to C should promote the formation of electron-rich late metal species that can readily undergo oxidative addition reactions. The trans-labilizing silyl donor was also expected to stabilize coordinatively and electronically unsaturated late metal complexes. The synthesis and reactivity of Ru complexes featuring bis(phosphino)silyl ligation of the type [?3-(2-R2PC6H4)2SiMe] ([R-PSiP]; R = Ph, Cy) are described. The 5-coordinate complex [Ph-PSiP]RuCl(PPh3) was shown to be catalytically active for the transfer hydrogenation of ketones in basic isopropanol. These transfer hydrogenation studies are among the first catalytic studies of silyl-pincer complexes and establish [R-PSiP]M species as viable candidates for catalysis. The synthesis and reactivity of 4- and 5-coordinate RuII complexes featuring the [Cy-PSiP] ligand were explored. Reaction of [Cy-PSiP]H with [(p-cymene)RuCl2]2 in the presence of NEt3 and PCy3 resulted in the formation of ([Cy-PSiP]RuCl)2, which serves as a precursor to a series of unprecedented 4-coordinate, formally 14-electron [Cy-PSiP]RuX (X = NHAr, N(SiMe3)2, OtBu) complexes that feature an unusual trigonal pyramidal geometry at Ru. The reactivity of these novel diamagnetic complexes is described, including the reaction of [Cy-PSiP]RuOtBu with amine-boranes resulting in the formation of rare bis(?-BH) complexes. Computational studies confirmed the key role of the strongly ?-donating silyl group of the Cy-PSiP ligand in facilitating the synthesis of such low-coordinate Ru species and enforcing the unusual trigonal pyramidal geometry. The mechanism of ammonia-borane activation was also examined computationally. Lastly, the synthesis and structural characterization of PdII complexes supported by the pincer-like bis(amino)phosphido ligand [?3-(2-Me2NC6H4)2P]- ([NPN]) is described. Examples of ?1-, ?2-, and ?3-NPN coordination to Pd are described, as is the catalytic activity of ([NPN]PdX)2 (X = Cl, OAc, OTf) complexes in the Heck olefin arylation reaction. In an effort to discourage the formation of phosphido-bridged dinuclear complexes, pre-coordination of the Lewis acid BPh3 to [NPN] was pursued. Upon reaction of [N(P?BPh3)N]K with [PdCl(C3H5)]2, the ?1-allyl complex [?3-N(P?BPh3)N]Pd(?1-C3H5) was isolated, which establishes the coordination of a Lewis acid to the phosphido donor of the [NPN] ligand as a viable strategy for encouraging the formation of mononuclear ?3-NPN complexes.

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Carbon-carbon bond formation is one of the most important reactions in organic chemistry, and the Suzuki-Miyaura cross-coupling reaction has become a forerunner in this area. Considerable research has been directed at the mechanistic aspects and synthetic utility of the reaction; however, little attention has been given to the formation of the putative PdL2 catalysts. Due to their high reactivities, these catalysts are typically difficult to store and therefore are often generated in situ in unknown yields and at unknown rates via any number of available palladium precursors. This thesis describes research directed towards determining the optimum conditions to quantitatively generate compounds of the type Pd(0)Ln (L = PMePh2, PPh3, PCy3, PMeBut2, PBut3, dppe, dppp, dppf) from Pd(h5-C5H5)(h3-1-Ph-C3H4). Pd(h5-C5H5)(h3-1-Ph-C3H4) has been found to be a superior precursor for synthesizing catalytically active PdL2 compounds due to its ease in handling and reactivity with tertiary phosphines. Furthermore, investigations into the role of water in the transmetallation step of the Suzuki-Miyaura reaction are presented. The research indicates that water is necessary to effect the transmetallation step when coupling [NBu4][PhBF3] with 4-bromotoluene in toluene; however, the amount of water above one equivalent has no significant effect on the rate or yield of the reaction. / Thesis (Master, Chemistry) -- Queen's University, 2009-07-23 12:57:10.248

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The performance of supported palladium catalysts that had been modified with triphenyl phosphine, diphenyl sulfide and triphenyl amine was compared to unmodified Pd/TiO2 catalysts. It was found that the selectivity was superior to the unmodified catalysts with or without the use of carbon monoxide as a modifier. Ethylene selectivities in excess of 70 % at conversion levels greater than 80% were observed on the modified catalysts under conditions of excess hydrogen which compares extremely well with other catalysts found in the literature. The modifier was found to significantly reduce the number of high energy sites on the surface on the palladium, thereby significantly hindering ethylene hydrogenation but not affecting acetylene hydrogenation. The simple preparation of these catalysts together with the simplicity added by not having to use carbon monoxide or limited hydrogen promises a viable alternative strategy for commercial catalysis.

This thesis deals with the palladium-catalyzed nucleophilic substitution of π-activated alcohols in which the C–O bond of a non-manipulated hydroxyl group is cleaved. The thesis is divided in two chapters describing two different catalytic systems. Chapter 2 describes a heterogeneous palladium-catalyzed transfer hydrogenolysis of primary, secondary, and tertiary benzylic alcohols to generate the corresponding aromatic hydrocarbons using formic acid as the hydrogen donor. A detailed mechanistic investigation of this reaction has been conducted that establish the kinetic order of each reaction component and also the deuterium kinetic isotope effects. This data provide a mechanistic picture that the hydride transfer from formic acid to palladium, and not the C–O bond cleavage, is involved in the rate-determining step and that a catalytic amount of a base promotes the transfer hydrogenolysis. Chapter 3 describes the development, mechanistic studies and synthetic scope of a homogeneous palladium-catalyzed amination of allylic alcohols. Isolation of the catalyst precursor and equilibrium studies of the palladium and π-acidic triphenylphosphite ligand show unique properties of this catalytic system. Stereochemical, kinetic, and kinetic isotope studies have been performed to provide insight into the mechanism of C–O bond cleavage of allylic alcohol and C–N bond formation catalyzed by the palladium complex. Interestingly, both O–H and C–O bond cleavages are involved in rate-determining steps.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1991. / Includes bibliographical references (leaves 309-319) / Microfiche. / xxiv, 319 leaves, bound ill. 29 cm

Proefschrift Universiteit van Amsterdam. / Met samenvattingen in het Nederlands en Frans.